The present invention relates to friction stir weld joints and, more particularly, relates to improving the material properties of a weld joint through interface preparation.
Friction stir welding is a relatively new process using a rotating tool having a pin or probe and a concave shoulder to join two workpieces in a solid state or to repair cracks in a single workpiece. For example, such a process is described in U.S. Pat. No. 5,460,317 to Thomas et al., the contents of which are incorporated herein by reference. During friction stir welding, the rotating probe is plunged into a workpiece or between two workpieces by a friction stir welding machine to produce the required resistance force to generate sufficient frictional heating to form a region of plasticized material. The probe is typically threaded to effect mixing of the plasticized material to thereby create a homogenous weld joint. This mixing action is particularly advantageous when welding workpieces formed of different materials. The tool is typically tilted at an angle relative to the workpiece or workpieces such that the trailing edge of the tool shoulder is thrust into and consolidates the plasticized material. Upon cooling of the plasticized material, the workpieces are joined along the weld joint. The magnitude of force exerted by the friction stir welding tool must be maintained above a prescribed minimum in order to generate the required frictional heating.
Friction stir welding is suitable for welding a variety of joint configurations, including butt joints, tee joints, corner joints, edge joints, lap joints and combinations of these. FIG. 1 illustrates a typical butt joint between two workpieces 11 wherein the faying surfaces of the two workpieces, i.e., the surfaces of the workpieces to be joined together, define an interface 13 that is in a plane parallel to the axis 15 of the friction stir welding tool 17. The friction stir welding tool 17 will plasticize the workpiece material proximate to the interface 13 and mix the material from side to side and top to bottom. The mixing action will remain fairly uniform and symmetric provided the welding parameters are maintained constant and the friction stir welding tool 17 remains on the centerline of the interface 13 defined by the two workpieces 11.
In contrast, as illustrated in FIGS. 2A-2E, at least some of the faying surfaces of workpieces 11 forming tee joints, edge joints, corner joints and lap joints, respectively, define interfaces 13 that are in planes generally perpendicular or transverse or both to the axis 15 of the friction stir welding tool 17. As illustrated in FIG. 3, the mixing action of the probe 17a of the friction stir welding tool 17 through a perpendicular or transverse workpiece interface 13 creates an uplift of material from the workpiece 11b farthest from the friction stir welding tool 17. The uplift of material results in a thinning of the workpiece 11a closest to the friction stir welding tool 17. In addition, the xe2x80x9cinterface notchxe2x80x9d 21 or portion of the workpiece interface 13 adjacent the weld joint 19 that is not consumed in the weld microstructure is typically moved with the uplifted material towards the edge of the workpiece 11a resulting in a stress concentration adjacent the weld joint. These conditions result in a weld joint 19 with low tensile and fatigue strength, and low fracture toughness.
Additionally, following friction stir welding, both the weld joint 19 and the heat affected zone, i.e., the portions of the workpieces 11a, b adjacent the weld joint, are more sensitive to corrosion attack from the ambient environment. As illustrated in FIG. 3, because the non-consumed portion of the interface 13 is not bonded, moisture can migrate down the interface and collect at the interface notch 21 adjacent the weld joint 19. This condition can result in a structural assembly having reduced corrosion resistance.
Thus, there is a need for an improved method of friction stir welding structural assemblies having interfaces that are perpendicular or transverse to the axis of the friction stir welding tool. The improved method should provide weld joints with high strength and fracture toughness, as well as other improved mechanical and chemical properties, including resistance to crack growth and corrosion resistance.
The present invention provides a method of strengthening a friction stir weld joint. According to one embodiment, the method includes forming a friction stir weld joint between first and second workpieces such that the first and second workpieces define at least one interface notch therebetween. As discussed above, the xe2x80x9cinterface notchxe2x80x9d is the portion of the workpiece interface adjacent the weld joint that is not consumed in the weld microstructure. Concurrently with the forming step, an interface layer positioned between the first and second workpieces is at least partially melted. The interface layer has a melting temperature lower than the solidus temperatures of the first and second workpieces so that, as the first and second workpieces are plasticized by the probe and shoulder of the friction stir welding tool, the interface layer melts from the heat generated through the friction stir welding process. The melted portion of the interface layer is then allowed to cool. Concurrently with the cooling step, the at least one interface notch is relocated away from the weld joint to thereby reduce the stress concentration adjacent the weld joint. In one embodiment, the weld joint is at least partially encased with the interface layer concurrently with the cooling step to thereby improve the corrosion resistance of the weld joint. In another embodiment, a boundary layer is formed adjacent to the weld joint, and wherein the boundary layer has a hardness less than the hardness of the weld joint to thereby improve the crack resistance of the weld joint. In yet another embodiment, the interface layer is partially diffused into the weld joint to thereby improve the mechanical and/or chemical properties of the weld joint.
The present invention also provides a method of manufacturing a structural assembly. According to one embodiment, the method includes providing a first workpiece defining a first faying surface. A second workpiece is provided defining a second faying surface. An interface layer is provided, the interface layer comprising a material having a melting temperature lower than the solidus temperatures of the first and second workpieces so that, as the first and second workpieces are plasticized by the probe and shoulder of the friction stir welding tool, the interface layer melts from the heat generated through the friction stir welding process. In one embodiment, the method includes placing the interface layer onto at least one of the first and second faying surfaces. In one embodiment, the placing step comprises positioning at least one layer of foil onto at least one of the first and second faying surfaces. In another embodiment, the placing step comprises at least partially coating at least one of the first and second faying surfaces with the interface layer. The first faying surface of the first workpiece is positioned adjacent to the second faying surface of the second workpiece such that the interface layer is positioned therebetween. The first and second workpieces are secured so as to prevent movement of the first workpiece relative to the second workpiece. A rotating friction stir welding probe is then inserted through the first workpiece and the interface layer and into the second workpiece to form a friction stir weld joint between the first and second workpieces and to at least partially melt the interface layer proximate to the probe to thereby increase the strength, corrosion resistance and fracture toughness of the weld joint. In one embodiment, the method comprises cooling the interface layer to at least partially encase the weld joint with the interface layer to thereby improve the corrosion resistance of the weld joint. In another embodiment, a boundary layer is formed adjacent to the weld joint, and wherein the boundary layer has a hardness less than the hardness of the weld joint to thereby improve the crack resistance of the weld joint. In still another embodiment, the interface layer is partially diffused into the weld joint to enhance the mechanical and/or chemical properties of the weld joint.
The present invention also provides a structural assembly comprising a first workpiece and a second workpiece. The first and second workpieces are positioned at least partially adjacent to each other so as to define an interface therebetween. In one embodiment, the first and second workpieces comprise dissimilar metals. In another embodiment, the first and second workpieces are formed of titanium, aluminum, AA 2000 series aluminum alloys, AA 5000 series aluminum alloys, AA 6000 series aluminum alloys, AA 7000 series aluminum alloys, aluminum-lithium alloys, ferrous alloys, bronze, and/or copper. In one embodiment, at least one of the first and second workpieces is comprised of an unweldable material, i.e., a material not generally weldable using conventional fusion welding techniques. The assembly includes a friction stir weld joint joining the first and second workpieces. In one embodiment, the assembly includes a plurality of friction stir weld joints joining the first and second workpieces. The assembly also includes an interface layer positioned between the first and second workpieces. The interface layer comprises a material having a melting temperature lower than the solidus temperatures of the first and second workpieces. In one embodiment, the interface layer has a melting temperature of less than about 500xc2x0 C. In one embodiment, the interface layer comprises a tin-based alloy or a zinc-based alloy. In another embodiment, the interface layer comprises ceramic particulate in a metal matrix. In another embodiment, the interface layer is multi-layered. In still another embodiment, the interface layer comprises a metal having an electrical conductivity of about equal to the electrical conductivity of at least one of the first and second workpieces. In still another embodiment, the interface layer has a thickness of between about 1 mil and about 5 mils. Advantageously, the interface layer at least partially fills the interface proximate to the friction stir weld joint to thereby increase the strength and fracture toughness of the weld joint, as well as other mechanical and chemical properties, including resistance to crack growth and corrosion resistance.
Accordingly, there has been provided an improved method of friction stir welding structural assemblies having interfaces that are perpendicular or transverse to the axis of the friction stir welding tool. The improved method will provide weld joints with high strength and fracture toughness, as well as other improved mechanical and chemical properties, including resistance to crack growth and corrosion resistance.